The present invention relates to a flexible tubular pipe for transporting hydrocarbons, of the type comprising at least one carcass, an inner sealing sheath, tensile armour plies and an outer sealing sheath. Such a pipe, commonly called a “rough bore” when the carcass constitutes the inner layer of the pipe, can in particular be used for transporting hydrocarbons in offshore oil production installations. It may optionally include, in addition to these layers, other special layers, such as a pressure vault, a hoop layer, intermediate sheaths, etc.
The carcass is, as will be recalled in the API 17J recommendations by the American Petroleum Institute, a tubular layer, in principle the inner layer, formed from an interlocked metal winding intended essentially to prevent the collapse of the inner sealing sheath or of the pipe in the absence of internal pressure in the pipe, owing to the effect of the external pressure, the pressure generated by the tensile armour plies or even the external mechanical loads (in particular in the gripping members when laying the pipe). Sometimes, the carcass may constitute an external layer in order to protect the outer surface of the pipe.
The carcass is conventionally made of a corrosion-resistant metal strip, generally having a profile having a shape, in cross section, similar to that of a recumbent S and wound so as to produce interlocked turns. Various proposals have already been made for improving the performance of a carcass. Thus, reference may be made to document EP 0 429 357 by the Applicant, showing a carcass whose constituent metal strip comprises a corrugation forming a bearing structure that increases the height of the cross section, and therefore the moment of inertia, improving the resistance of the carcass to collapse. According to document FR 2 780 482, also in the name of the Applicant, the mechanical properties of the carcass are improved by carrying out a prior work-hardening operation on the constituent strip.
In the case of current carcasses made of interlocked (recumbent S) strip of the type described in document EP 0 429 357, the mechanical properties governing resistance to collapse are limited for a given pipe diameter. This is because the diameter of the pipe imposes, in the case of the S-section in question, a maximum thickness of the strip that can be used. This thickness is limited, in particular because of the S-section, which it is impossible to wind over a given diameter for strip thicknesses greater than the limiting thickness. Since the moment of inertia of the S-section producing the carcass depends in particular on the overall thickness of the latter, and therefore on the thickness of the strip used, this thickness has a maximum for a given diameter (and therefore there is a maximum thickness of the strip that can be used). Consequently, for a given pipe diameter, the carcass that can be used poses a limitation on its collapse resistance (inertia) characteristics. Since the design of a carcass depends directly on the hydrostatic pressure that exists at the depth at which the pipe is used, the latter being designed to be able to withstand the hydrostatic pressure that there would likely be in the annulus in the pipe if the sheath were punctured, each pipe diameter poses a limitation on the possible depth at which the pipe can be used, this limitation being tied to the strength of its carcass.
Moreover, such carcasses produced from interlocked S-sections also have other problems, these being in particular:                an inertia problem associated with a mean fill factor;        a cost problem associated with the cost of the raw material employed (generally stainless steel in order to be corrosion-resistant); and        a head loss problem due to the gaps, which may be substantial, between adjacent turns of the winding and which create discontinuities in the surface of the internal face of the carcass. These problems are greater when it is desired to have a high inertia and when a thick strip is used.        
There are alternatives to winding a strip in order to produce a carcass, especially using Zeta or Teta-shaped wires interlocked by a U-section, as shown in document FR 2 772 293. However, the latter solution proves to be expensive and poses other problems, in particular a weight problem when the pipe is used at great depth.
There are also in the prior art carcasses made from spiralled wire, but these are in no way suitable for the type of applications envisioned here, especially on account of their inability to provide a local minimum inertia and therefore inability to ensure that the carcass has the desired resistance to collapsing.
Thus, the known carcasses of the prior art are unsatisfactory when it is desired to produce pipes intended to be used at great depth. They have problems either of cost and weight (profiled wire) or of a limitation in the diameter of the pipe that can be used for a given depth (interlocked S-shaped strip), as well as head loss problems.